1

Authentication Protocols

Celia LiComputer Science and Engineering

York University

2

Outline Overview of authentication Symmetric key authentication protocols (Kerberos) Public key authentication protocols (SSL) Authentication protocols in Wireless Local Area Networks (WLAN)

3

Outline Overview of authentication Symmetric key authentication protocols (Kerberos) Public key authentication protocols (SSL) Authentication protocols in Wireless Local Area Networks (WLAN).

4

Overview of AuthenticationThe process of determining whether someone is, in fact, who it is declared to be.

An authentication protocol consists of a sequence of messages between principals and will be described using different notations.

Principals: parties who cooperate by exchanging messages over networks.

5

Notation E(K : M): denotes encrypting message plaintext M with key K. Ka: key of principal A  Kab: shared key of principle A and B

Principals are generally denoted by capitals such as A, B and S (server)

(1) A B : M1 (2) B S : M2 (3) S B : M3

6

Notation

Na: A number generated by a principal A. Generally termed as a nonce.

A nonce could be • a timestamp: a number denoting the current time.• a sequence number• a random number  A B : A, E(Kab : Na)

7

Overview of Authentication Timeliness in Authentication Protocols

An authentication protocol is required to guarantee that the parties involved in the authentication process are present during the execution of the protocol.

Nonces are generally used to assess timeliness in authentication protocols.

8

Attacks on Authentication Protocol

Replay attack: The attacker records the message of an authentication protocol and replays this information to attempt to falsely authenticate to the other principle.  Counter measures: Using timestamps, sequence number or random numbers. 

9

Overview of authentication Symmetric key authentication protocols (Kerberos) Public key authentication protocols (SSL) Authentication protocols in Wireless Local Area Networks (WLAN)

Outline

10

Symmetric Key Authentication Without Trusted Third Party

Symmetric key one-pass unilateral authentication protocol

Na: Timestampt , Ra, Rb: Random numbers.

B authenticates A:

Symmetric key two-pass unilateral authentication protocol

Symmetric key three-pass mutual authentication

If Na (timestamp) has a recent value, B can prove A’s identity.

B authenticates A:

B authenticates A:

A authenticates B:

11

Symmetric Key Authentication With Trusted Third Party

S: Trusted Third Party

A B

• A gets a shared key Kab generated by S

A & B trust S, Kas is a shared key of A & S, Kbs is a shared key of B & S

• B gets shared key Kab

• A authenticates B

• B authenticates A

(1)(2)

(3), (5)

(4)

NS (Needham-Schroeder) Symmetric Key Authentication

12

Kerberos Authentication

A typical example of symmetric key authentication with trusted third party The trusted third party in Kerberos is the

authentication server which is in charge of

distribute shared keys. Kerberos ticket is employed for the

authentication purpose

13

Kerberos Authentication

Kerberos Ticket A certificate issued by an authentication server, encrypted using the key of the authentication server. The ticket contains

the key that will be used for authentication of the client to the verifier who will verify the correctness of the key the name of the principal to whom the key was issued an expiration time of the ticket

14

Kerberos authentication

Client C wants to access server S. Client C and sever S need to authenticate each other Authentication server is comprised of two parts:

A: Key Distribution Server G: Ticket Grant Server

(1) C sends a random number N1 to A (2) A sends back a ticket Tcg to C.

Kac: shared key between A and C Kcg: shared key between C and GC authenticates A if N1 is correct

15

Kerberos authentication (3) C forwards the ticket Tcg to G

N2: random number T: time stamp G authenticates C if time stamp is correct

(4) G sends back a service ticket Tcs to C. Kcs: shared key between C and S C authenticates G if N2 is correct

(5) C sends ticket Tcs to S T’: time stamp S authenticates C if time stamp is correct

(6) C authenticates S if time stamp is correct

16

Overview of authentication Symmetric key authentication protocols (Kerberos) Public key authentication protocols (SSL) Authentication protocols in Wireless Local Area Networks (WLAN)

Outline

17

Public Key Certificate

Commonly used for public key authentication protocols An electronic document which uses a digital signature to bind together a public key with an identity:

the name of a person or an organization, their address

Issued by a certification authority (CA) e.g. VeriSign

The certificate can be used to verify if a public key belongs to an individual.

18

Public Key Certificate

Public key certificates include: Public key of the user DN (name of the user) Validity period of the certificate Name of the certificate issuer Digital signature of the certificate issuer

19

Public Key Authentication without Trusted Third Party

Public key one-pass unilateral authentication protocol

Public key two-pass unilateral authentication protocol

Public key three-Pass mutual authentication protocol

CertA: public key certificate of A

Ka-1: private key of A, Kb-1: private key of b

B authenticates A:

B authenticates A:

B authenticates A:

A authenticates B:

20

Public Key Authentication with Trusted Third Party

NS (Needham-Schroeder) Public Key Authentication Protocol

S: Trusted Third Party

A B

B authenticates A by verifying Nb

A authenticates B by verifying Na

S sends B’s public key Kb to A

S sends A’s public key Ka to B

B asks A’s public key from S

B gets A’s identity

A asks B’s public key from S

(1)

(2)(3)

(4)(5)

(6)

(7)

21

Secure Socket Layer Protocol (SSL)

• Universally accepted on the Web for authenticated and encrypted communication between clients and servers.

• Typical example of public key authentication

• Protect sensitive information such as Social Security Number (SSN), Credit Card Number, etc.

Web Server Client

22

SSL Server Certificate

The SSL protocol requires a server certificate so that client can authenticate the server.

Client Certificate (Optional)

You can optionally configure your server to request a client certificate so that server can authenticate the client.

Only discuss how client authenticates server in SSL

23

SSL 1. Client server: Hello message

2. Server client: server’s public key certificate, including server’s public key, digital signature signed by certificate authority (CA)

3. Client uses CA’s public key to verify digital signature signed by CA, thus verifies server’s certificate.

4. Client server: a random number

in order to prove server’s identity.

5. Server encrypts random number with its private key.

6. Server client: the encrypted message

7. Clients decrypts the message using server’s public key to verify identity of the server.

Client authenticates server based on certificate issued by certificate authority (CA)

24

Compare SSL and Kerberos

SSL KerberosUses public key encryption Uses symmetric key

encryptionCertificate based   Relies on a trusted

third party  Ideal for secure communications with a large, variable user base that is not known in advance, such as the WWW. 

Ideal for networked environments where all services and users are known in advance.

25

Overview of authentication Symmetric key authentication protocols (Kerberos) Public key authentication protocols (SSL) Authentication protocols in Wireless Local Area Networks (WLAN)

Outline

26

Authentication Protocols in WLAN WPA (Wi-Fi Protected Access) Certification program created to secure wireless computer networks.• Provides stronger data encryption and user authentication• Implements the IEEE 802.11i standard.  The three entities of WPA are the client, the Authentication Server (AS), and the Access Point (AP).

• Only after the client is authenticated by the authentication server, the authenticator (Access Point) allows the client to access the network

AS

27

Authentication Protocols in WLAN Symmetric key approach: EAP-LEAP Public key approach: EAP-TLS  Symmetric Key Approaches Efficient and require little computational power

Resource constrain, e.g., computational power of PDAs, mobile VoIP phones, have.

Drawbacks Most protocols derive the shared secret from the user's password and some passwords are not strong, it is easy for the attacker to extract the secret key from them.

28

Lightweight Extensible Authentication Protocol (LEAP)

The server knows the password of the client. Password is the shared key between the client C and authentication server S

29

Authentication Protocols in WLAN

Public key Approaches Provides strong security Cannot authenticate a client without public certificate Example: EAP-TLS

30

EAP-Transport Layer Security (EAP-TLS)

• AS verifies client’s digital signature using client’s public key got from client’s certificate Certclient

• Get random number p by decrypting with its private key

• Client calculates H(c,s,p), compares it with the value sent by As

31

References

[1] Kerberos http://web.mit.edu/kerberos/www/

[2] SSL http://docs.sun.com/source/816-6156-10/contents.htm

[3] EAP-TLS http://en.wikipedia.org/wiki/Extensible_Authentication_Protocol